Communications devices such as terminals are also known as e.g. User Equipments (UEs), mobile terminals, stations (STAs), wireless devices, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a wireless communications network, such as a Wireless Local Area Network (WLAN) or a cellular communications network sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via an access network and possibly one or more core networks, comprised within the wireless communications network.
The above communications devices may further be referred to as mobile telephones, cellular telephones, laptops, tablets or sensors with wireless capability, just to mention some further examples. The communications devices in the present context may be, for example, portable, pocket-storable, hand-held, wall-mounted, computer-comprised, or vehicle-mounted mobile devices. The communications devices are enabled to communicate voice and/or data, via an access network, such as a Radio Access Network (RAN), with another entity, such as e.g. an Access Point (AP), another communications device or a server.
The communications network covers an area, e.g. a geographical area, which is divided into subareas, such as service areas, coverage areas, cells or clusters. In a cellular communications network each cell area is served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNodeB (eNB), NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB, micro eNode B or pico base station, based on transmission power, functional capabilities and thereby also cell size. A traditional cell is the area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the communications devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the communications device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the communications device to the base station.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems.
Ultra-Lean System Design of NeXt Generation (NX)
A design principle currently under consideration for the NX generation communications networks, also known as Next Radio or New Radio (NR) in 3GPP context, is to base it on an ultra-lean design. This implies avoidance of “always on signals” from the communications network as much as possible. Some examples of benefits from such design principle is expected to be a significantly lower network energy consumption, a better scalability, a higher degree of forward compatibility during the Radio Access Technology (RAT) evolution phase, a lower interference from system overhead signals and consequently higher throughput in low load scenario, and an improved support for user centric beam-forming.
Heavy Use of (Massive) Beam-Forming
Advanced Antenna Systems (AAS) is an area where technology has advanced significantly in recent years and where a rapid technology development in the years to come is foreseen. Hence it is natural to assume that advanced antenna systems in general and massive Multiple Input Multiple Output (MIMO) transmission and reception in particular will be a cornerstone in a future NX communications network.
Mobility Reference Signals
In deployments with large antenna arrays and many possible candidate beam configurations, all beams cannot transmit signals in an always-on, static manner for the sake of mobility measurements. Instead, the connected access nodes select a relevant set of mobility beams to transmit when required. Each mobility beam carries a unique Mobility Reference Signal (MRS). The communications device, e.g. the UE, is then instructed to measure on each MRS and report information relating to the performed measurement back to the communications network, e.g. to an access node. Based on some criteria, for example a difference between MRS strength between two mobility beams, a handover can be triggered. For mobility to work efficiently, the involved Access Nodes (ANs) need to maintain beam neighbour lists, exchange beam information, and coordinate MRS usage.
Access Node, e.g. Base Station, Relations
Despite advanced radio network planning tools, it is very difficult to predict the radio propagation in detail. As a consequence, it is difficult to predict which base stations that needs to have a relation with each other and maybe also a direct connection with each other prior to the network deployment. This has been addressed in LTE, where UEs could be requested to retrieve unique information from the system information broadcast of unknown base stations and report to the serving base station. Such information was used to convey messages to the unknown base station via the core network, which maintained a lookup table from a unique identifier to an established S1 connection. One such message was used to request transport network layer address information necessary for a direct base station to base station connection for the X2 interface. In order for smooth operations of the mobility procedure in the NX generation, the NX node needs to have a concrete list of neighbouring NX nodes which can be handover candidates for the UEs.
Active Mode Mobility (AMM)
When a communications device, e.g. a UE, moves in a service area, there might be a need to change the serving node and/or the serving beam in order to maintain a reasonable radio link between the UE and the wireless communications network.
A first way of handling Active Mode Mobility (AMM) is to use downlink reference signal transmissions similar to what is done in LTE communications networks of today. More precisely, each mobility beam carries a unique Mobility Reference Signal (MRS). The UE is then instructed to measure on each MRS and report to the wireless communications network. Based on some criteria, for example a difference between MRS strengths between two mobility beams, a handover can be triggered. However, as opposed to the LTE communications networks, in deployments with large antenna arrays and many possible candidate beam configurations, all beams cannot transmit reference signals in an always-on, static manner for the sake of mobility measurements. Instead, the connected Access Nodes, e.g. base stations such as eNBs, select a relevant set of mobility beams to transmit when required. For the mobility to work efficiently, therefore, the involved ANs need to maintain beam neighbor lists, exchange beam information, and coordinate MRS usage.
A second way to handle AMM is to use uplink based solutions for AMM management. The idea is to configure the UE to transmit a reference signal, denoted an Uplink Synchronization Signal (USS) here after, when the radio link deteriorates. The transmission of the USS may be triggered by either the UE or the communications network. One or more neighbor network nodes then measure the signal strength of the USS transmission and report it to the serving node. Based on the measurement report, the best beam is chosen and the UE is handed-over to the corresponding node.
A third way for handling AMM is to combine the downlink and the uplink based AMM mobility solutions into a so called hybrid solution for AMM management. The hybrid solution has a lower latency as compared to each of the downlink and uplink based solutions described above. In one alternative of the hybrid scheme, there is one USS reserved at the source node for each of the MRS transmissions. In case there is a need for beam switch, the MRS:s are transmitted by the candidate beams. The UE measures the signal strength of each MRS transmission, determines the best beam and reports it to the corresponding node via the provided USS of the beam. The serving node, based on the receipt USS, determines the target node and a Handover (HO) is initiated. Alternatively, the candidate nodes may reserve the USS for each MRS that they are transmitting and hence the UE, by measuring of the signal strengths of the MRS transmissions, is able to pick the strongest beam and send the USS of the corresponding beam which explicitly indicates the target node too. The corresponding network node would then reply with a random access response indicating that it will admit the UE.
In case the UE does not receive any response after some period of time in each of the alternatives described, it will pick the second strongest beam and follow the same procedure. The period of time may be predetermined and/or it may be fixed or adjustable.
A drawback with prior art solutions, such as the LTE solution, for establishing neighbour node relations is that they are based on the transmission of always-on signals, e.g. always-on reference signals. But the always-on signaling is absent or very sparse in a NX communications network by design and therefore are not very useful when establishing neighbor relations in a NX communications network which thus requires a different approach compared to the existing LTE solutions.